The art has recognized for many years that instruments can be made, to measure the level of materials in a vessel, which operate on admittance responsive principles, whereby the admittance between a probe in a vessel and the vessel is measured, typically by driving the probe with an rf signal and comparing the admittance of the probe to a reference admittance, e.g., in a bridge circuit. Many prior art patents show this subject matter generally, including commonly-assigned U.S. Pat. Nos. 4,146,834 and 4,363,030. Typically, calibration of these prior art systems has been performed manually; the level of materials in the vessel is controlled to bear some predetermined relationship to the probe, and the reference admittance carried by the transmitter at the vessel is manually adjusted until the transmitter's output signal is appropriate for the level of the materials. Frequently, plant conditions make it difficult or hazardous for a person to gain access to the transmitter to perform calibration manually, so that while this approach is workable, it would be desirable if calibration could be accomplished in a more efficient fashion; in particular, it would be desirable to provide a system in which calibration could be accomplished automatically and remotely, to eliminate the necessity of an operator's having to physically approach the transmitter.
It is accordingly an object of the invention to provide a remotely-calibratable admittance-responsive instrument system.
The art has generally standardized on so-called two-wire instrument systems, in which a receiver including a power supply is connected to a remotely-located transmitter by a pair of conductors which both supply power to and convey the output signal from the transmitter. The transmitter draws current in a predetermined range for its operations; the amount of current drawn by the transmitter provides the output signal.
It is an object of the invention to provide a remotely-calibratable two-wire instrument system, in which a transmitter located at a vessel draws a current responsive to the level of materials within the vessel.
In many plant situations, the transmitters used for level sensing are not readily accessible. Therefore, it would be desirable to provide a calibrator for calibrating such a system which could be attached to the transmission line connecting the transmitter and receiver, preferably at any point thereof, to enable the maximum flexibility in calibration operations, and to provide such is an object of the invention.
Some prior art admittance-responsive measurement systems have included remote or automatic calibration schemes. Typically these have involved comparison of a variable signal, such as the admittance measured by a probe, to a fixed reference admittance, and adjustment of some circuit component until the actual admittance signal sensed by the probe bears a predetermined relationship to the reference signal which calibrates for an air capacitance as well as preload. Thereafter it is assumed that no further variation in the circuit takes place. It will be appreciated that this is not necessarily the best approach inasmuch as the reference admittance may not be correct, may itself drift over time, and, furthermore, that this does not permit ready variation of the reference admittance with variation in the materials, the level of which is to be measured, for example.
Accordingly, it is an object of the invention to provide a remotely-calibratable level-measuring system in which the reference admittance is established during a remotely-initiated calibration operation, by variation of the reference admittance compared to an actual admittance signal determined in response to the existence of a predetermined condition of the materials (for example, the materials being at a level beneath any probe used), such that the correct reference admittance is determined regardless of the characteristics of the material being measured.
Some prior art admittance responsive measurement systems are known in which a reference admittance is varied and compared to an actual admittance signal in a calibration operation in accordance with this object of the invention. However, other aspects of these systems are not as useful as would be desired. For example, it is usual to provide "preload" in level-measurement systems. "Preload" is the difference between the operating point admittance (i.e., the admittance at which an associated control system responds) and the air capacitance (i.e., the admittance sensed with respect to an empty vessel). Thus, for example, the instrument may detect an admittance of 20 pf when the materials are above the level of a probe in their vessel and 10 pf when the materials are well below the level of the probe. It would be desirable to set the instrument to change output states (e.g. to cause the vessel to be filled) at 15 pf (i.e., preload=5 pf)--which is halfway between the full and empty capacitances. This choice of operating point would minimize the possibility of false indications by the transmitter due to minor variations in characteristics of the material and due to transmitter performance variations over time. To date, all remotely calibratable instrument systems of which the present inventors are aware require selection of preload at the transmitter itself, thus defeating the aim of remote calibration.
It is therefore an object of the invention to provide a remotely-calibratable admittance responsive instrument system in which preload may be established remotely as well.
It is a further object of the invention to provide a remotely-calibratable admittance sensitive instrument system in which a number which corresponds to the value of reference admittance, once having been determined, is retained in non-volatile storage means, so that the instrument remains calibrated during power outages and the like.
In addition to the non-volatility of the stored calibration information, it would similarly be desirable if the calibration information was secure from tampering or from spurious environmental influences, and such is additionally an object of the invention.
It is a further object of the invention to provide a remotely calibratable instrument system in which the calibration operation is performed in as simple and foolproof a manner as possible.
It is an object of the invention, therefore, to provide a remotely-calibratable instrument system in which the calibration operation is accomplished automatically by the operator's pushing a single button (preload selection and calibration mode switches having been set), whereupon calibration is accomplished automatically with respect to a known predetermined condition of materials, such that a correct reference standard is derived for use in subsequent comparison to actual level signals, as opposed to systems in which it is assumed the reference signal is never to be varied.
It would be desirable if the same piece of calibrating hardware, used to initiate calibration operations in which a reference admittance is varied and compared to an admittance sensed with respect to a known condition, could be made useful for calibration of a number of such transmitters, so that the hardware costs could be shared over a larger system.
It is therefore an object of the invention to provide a remotely-calibratable admittance sensitive level measurement system, in which a calibrating device is adaptable to calibrating a large number of differing transmitters.
As mentioned above, it is desirable that the transmitter and receiver be connected by a two-wire transmission line. For simplicity's sake, it would be desirable if the calibrator could be detachably connected for calibration to the same two-wire line, preferably at any point between the transmitter and receiver, and such is accordingly an object of the invention.
The invention has been described heretofore in connection with a two-wire system in which two wires connect a receiver, comprising a power supply and load, and a transmitter, the calibrator being adapted to be attached to the pair of wires. It will also be appreciated by those of skill in the art that, in some circumstances, "receivers" are not necessary. Instead, the transmitter comprises means for controlling a local condition, e.g., a transmitter monitoring the level of materials in a vessel may comprise means for controlling the flow of materials into the vessel. Similarly, if the transmitter is used only for providing a condition indication, no discrete receiver is required. Further, the transmitter may be connected to a local power supply, such that it is not connected to a receiver by a two-wire system. Nevertheless, all such transmitters require calibration, and it would be desirable if the same calibrator used to calibrate such receiver/transmitter systems could be used to calibrate such receiverless systems.
It is accordingly an object of the invention to provide a remotely calibratable transmitter in a receiverless system. The term "receiverless system" as used herein, of course, includes a power supply although it does not otherwise include a receiver.
It will be appreciated that a calibration-enabling signal, sent from the calibrator to the transmitter to cause initiation of a calibration operation, must be distinguishable by the transmitter from noise, accidental short or open circuits and the like occurring in the transmission channel.
It is therefore an object of the invention to provide a remotely-calibratable instrument system in which a calibrator transmits an encoded calibration enabling signal and in which a transmitter is adapted to decode the encoded signal and initiate calibration operations in accordance therewith.
Having determined that remote calibration would be desirable, it is furthermore desirable that such a remote calibrator be as versatile and as useful as possible. For example, in the preferred embodiment, in a calibration operation, the variable capacitance used by the transmitter as the reference admittance is caused by the calibrator to be incremented through a number of predetermined steps; in each step, the reference capacitance is compared to the admittance sensed by the probe in response to a given condition of materials for detection of a predetermined relation therebetween. It would be desirable if the number of the step at which the varied reference admittance bore the predetermined relationship to the actual admittance were displayed on the calibrator in readable form for the use of the operator for future reference and, in some cases, to determine the appropriate preload. To provide such a display is an object of the invention.
Similarly, it would be desirable if the remote calibrator were adapted to cause the transmitter to transmit its setting to the calibrator, to enable confirmation of its setting at the correct reference admittance value, such that an operator could remotely calibrate a transmitter, make a note of the reference admittance value selected, and be able to return some time later and use the calibrator to cause the transmitter to output its reference level in order to check that the proper calibration value was set in the transmitter. Accordingly, provision of a remote calibrator with this capability is an object of the invention.
It would further be desirable to provide a remotely-calibratable instrument system in which the calibrator could be operated to cause the transmitter to vary its reference signal a predetermined amount from a known starting value, regardless of the conditions of materials in the vessel, and to provide such a system is accordingly a further object of the invention.